The Nitty-Gritty of Green Roofs

When the City of Toronto passed a bylaw in May 2009 requiring all new buildings with a gross floor area larger than 2,000 square meters to be built with a green roof, Liat Margolis saw her opportunity. The following year she launched the Green Roof Innovation Testing Laboratory, or GRIT Lab, a windswept research perch on the rooftop of the University of Toronto’s John H. Daniels Faculty of Architecture, Landscape, and Design building. Here, a multidisciplinary team of landscape architects, civil engineers, building scientists, and biologists study nearly every metric associated with the environmental performance of green roofs—from storm-water retention and evaporative cooling to biodiversity and life cycle costs.

“We’re studying the synergy of the system for its capacity to deal with both water and thermal energy, as well as how these cycles affect plant growth and vice versa,” says Margolis, an assistant professor of landscape architecture and the project’s principal investigator.

Before the GRIT Lab, Margolis served as director of the Materials Collection at the Harvard Graduate School of Design and director of research at Material ConneXion, where she developed a materials database and consulted for a broad range of design firms and manufacturers. She’s a living systems luminary who speaks beyond property boundaries and municipalities to parks and river systems that are “the muscle tissue and veins of any region.” For her, the GRIT Lab project stands at the heart of a larger ecological discussion of urban infrastructure management in the face of climate change.

“In some cases, green roofs are studied in biology or engineering departments with attention to plant growth, water balance, and cooling as isolated subjects,” Margolis says. “The key aspect of this initiative is its multidisciplinary collaboration.”

Dialogue Liat Margolis

gb&d: Was this a natural extension of the research you did at Material ConneXion or a leap into something new? That is, how did you become interested in green roofs with your background in materials?

Liat Margolis: To me they are still materials. Material in the most basic way is a composition of elements. It has certain properties. For instance, if you look at a tree or a wooden beam or a sheet of paper, these are all material systems that have to do with natural systems and anthropogenic cycles, in which a tree becomes a sheet of paper, or conversely, if placed in the fireplace, it becomes fire. We try to organize and reorganize them to manage our built environments in whatever way best suits our needs.

The only leap in my recent academic position has been the opportunity to investigate materials in a very hands-on way. As specifiers of material, designers are rarely involved in the actual design of materials or their study. In the context of green roofs, I want so see how all the systems work, evaluate them, and influence the direction the industry is headed next.

gb&d: Okay, so how do green roof materials differ from those used in other built environments?

Margolis: The main principle is to think of a green roof as a system in constant flux rather than a fixed product. It is a set of standards that we need to customize for specific ecological and climate regions and management priorities. Green roofs cannot be conceived in the same category as other building products, like bricks and mortar—they are a living system that is part of a larger hydro-ecological system. Because we live in cities that expand rapidly and that have lost enormous areas of porous surfaces, we have to either protect ourselves from weather systems or harness their energy for constructive purposes.

gb&d: How do you see the role of the designer or landscape architect evolving in the future of green roofs?

Margolis: Ultimately, as academic researchers we don’t have a huge stake in the financial outcome of the green roof business. There is more at stake for industry, which has to withstand the financial consequences of changes to policy and standards. The dilemma lies in the gap between the economic reality of industry and policy on one hand, and aspirations toward effective environmental technologies on the other. Designers should not accept the one-size-fits-all environmental claims of prevailing “green” technologies de facto but take a more critical approach to current green-building standards, going beyond lip service toward the optimization of sustainable design.

Wiring a Living Lab

With a view of the famed CN Tower jutting from Toronto’s skyline, the GRIT Lab’s 33 test beds are arranged in raised, 4-by-8-foot, membrane-coated, flashed-wood boxes that resemble miniature wildflower meadows or xeric habitats. Tawny grasses and low-growing herbaceous plants are planted in lightweight, 4- to 6-inch-deep soil-less growing media and monitored by 270 sensors connected to more than 5,000 linear feet of wiring. Data is collected constantly. Each bed is equipped with thermal and moisture sensors, a rain gauge, and an infrared radiometer. These systems record soil moisture, effluent flow rates, and temperature measurements every five minutes and analyze them against climate data from an on-site weather station.

But all the raw data is worthless without an ability to measure the impact to plant growth, water balance, and thermal cooling performances. This is where the school’s masters and PhD candidates come in. Their work—recording weekly observational notes about plant cover, phenology, and succession as well as monitoring and analyzing the sensor data—is supported by more than $500,000 in grant funding, as well as in-kind contributions from industry partners, such as Tremco, Bioroof Systems, IRC Group, Flynn, Siplast, DH Water Management, Toro, and STLAi.

One of project’s major goals is to study how green roofs mitigate urban heat island effect by cooling the air surrounding buildings. Because of the prevalence of dark roof membranes and non-reflective paved surfaces that absorb solar radiation and release it back in the form of heat into the atmosphere, cities tend to be warmer than forested or vegetated environments. Green roofs help counteract this phenomenon through a biological process known as evapotranspiration; plants act as an external air-conditioning system, absorbing rainwater through their root systems and releasing cooler air to the surrounding environment through stomata in their leaves.

Researchers are also evaluating how growing media and depth optimize storm-water retention. As older cities such as Toronto, Chicago, Boston, and New York City have expanded and added impervious surfaces, their combined sewer and storm-water systems, many a century old, have become unfit for handling peaks in water volume. Extreme weather events, such as torrential rains, tornadoes, hurricanes, that are more common as a result of climate change can cause these systems to overflow, leading to significant structural damage and contamination of the water supply. Green roofs relieve pressure on these systems by acting as natural sponges, soaking up rainwater before it finds its way into storm drains.

Media Bias

It’s remarkably difficult to determine which growing media provides the greatest water retention while supporting healthy plant growth and ensuring the highest rate of evaporative cooling. A product approved by the German Landscape Research, Development and Construction Society (the German name is abbreviated as FLL), a nonprofit plant research organization, is widely considered the industry standard. The FLL popularized a non-organic, lightweight, low-maintenance, mineral aggregate similar to gravel or lava rock. It puts minimal strain on the load capacity of building roofs, requires little upkeep, and provides an adequate substrate for sedum, the drought-tolerant succulent that is a favorite among green roof designers.

Sedum undoubtedly has its advantages because it evolved in a water-scarce environment and requires little water and nutrients to survive, but it has one drawback—it evapotranspires at night, not during the day when cooling is most important. In addition, the FLL soil media may not have the optimal retention capacity for increasingly heavy rains. So, in addition to evaluating the above components, the GRIT Lab is testing two alternatives: a 40–50 percent organic growing media, developed by Bioroof Systems, and a biodiverse meadow planting mix, both of which are thought to have higher water-retention capacity and evapotranspiration rates.

Although Margolis cautions that the study is in its beginning phases, she is observing distinct differences among test beds with respect to plant growth and associated growing conditions. She has found that irrigation has been critical for plant diversity but less important for plant cover and biomass; growing media type has proved significant for plant cover but not for plant diversity; sedum was significantly less affected by changes to growing media and irrigation than non-sedum; and meadow species planted in a 40–50 percent organic media display much higher plant cover than meadow species planted in FLL’s non-organic soil media.

The embedded energy costs associated with green roof materials are an important part of the study. “The embedded energy cost of the high-organic growing media we are testing is relatively low because it utilizes a waste product which would otherwise go to a landfill,” Margolis says. “In contrast, FLL aggregate is quarried, processed, and sometimes transported great distances. For instance, in the case of Toronto, the aggregate is typically sourced from the US or Quebec, which translates to high energy costs.”

Next: Solar Synergies

In addition to the industry partners associated with phase one of the GRIT Lab project, Sky Solar (whose vice president Helen Platis was instrumental in pulling the project together), Schletter, Semple Gooder, and TerraGen Solar are contributing to a second phase of the project, which from 2013 to 2016 will investigate the hypothesis that green roofs and photovoltaic solar panels exist in a symbiotic relationship that magnifies the sustainability benefits of each. Several studies have shown that reducing the operating temperature of photovoltaic panels increases their conversion efficiency by as much as 0.5 percent per 0.9 degrees Fahrenheit, and lowering operating temperatures minimizes the degradation of the panels.

The thinking is that installing photovoltaic arrays above a vegetative surface will allow plants to cool the solar panels through evapotranspiration and solar reflectance, which in turn will improve solar energy production and increase the potential return on investment for green roof developers and owners.